These are directions posted by the members of the Disboutique over on the Disboards. I have compiled them here for easier reference.

Wednesday, September 10, 2008

Chart for Sizing DOWN a Pattern by CarlaC

Origianally posted by LouiesMama (CarlaC ) Thank you Carla!!!
http://www.disboards.com/showpost.php?p=27480030&postcount=1950
My patternmaking book only goes down to size 3, so I used the size charts
in "Simply the Best Sewing Book" by Simplicity.
Their measurements for each size (chest/waist/hip) in inches are as
follows:
0.5 - 19/19/20 (six months)
1 - 20/19.5/21
2 - 21/20/22
3 - 22/20.5/23

If you are using a commercial pattern, you might want to check the
measurements they use for each size just to make sure they correspond (if the
measurements aren't on the pattern, you can find them in the pattern book or on
the company's website). So here are the numbers I came up with:
Size 3 down to size 2 - scaling set to 95%
Size 3 down to size 1 - scaling set to 91%
Size 3 down to size 0.5 - scaling set to 86%
Size 2 down to size 1 - scaling set to 95%
Size 2 down to size 0.5 - scaling set to 91%
Size 2 down to size 0 - scaling set to 86%
Size 1 down to size 0.5 - scaling set to 95%
Size .5 down to size 0 - scaling set to 90%

I based the newborn size 0 on a chest measurement of 18" (this one is for
you, T ). So it seems like (at least for the tiny sizes) the general rule of
thumb is:
Shrinking 1 size - scaling set to 95%
Shrinking 2 sizes - scaling set to 91%
Shrinking 3 sizes - scaling set to 86%